The invention relates to medical devices (articles) formed from improved materials that include pyrolytic carbon as a component of a composite. In particular, the invention relates to medical devices that incorporate improved composite materials including a pyrolytic carbon component and a metal/metalloid carbide component.
A variety of medical devices are designed particularly for contact with a patient's bodily fluids. The duration of this contact may be relatively short, as is typical with surgical instruments, or may be long term, as is typical with prosthetic heart valves implanted into the body of a recipient, and other implanted prostheses. Some devices, such as catheters, can have either short term or relatively long term contact.
Prostheses, i.e., prosthetic devices, are used to repair or replace damaged or diseased organs, tissues and other structures in humans and animals. Prostheses generally must be biocompatible since they are typically implanted for extended periods of time. Examples of prostheses include, without limitation, prosthetic hearts, prosthetic heart valves, ligament repair materials, vessel repair and replacement materials, stents, and surgical patches.
Physicians use a variety of prostheses to correct problems associated with the cardiovascular system, especially the heart. For example, the ability to replace or repair diseased heart valves with prosthetic devices has provided surgeons with a method of treating heart valve deficiencies due to disease and congenital defects. A typical procedure involves removal of the native valve and surgical replacement with a mechanical or bioprosthetic, i.e., tissue based, valve. Another technique uses an annuloplasty ring to provide structural support to the natural annulus of the native valve. Annuloplasty rings can also be used with prosthetic heart valves.
Many biocompatible medical devices and/or their components have significant requirements with respect to their mechanical and physical properties. For example, the medical devices are often limited in their size. At the same time, the devices and/or their components may be subjected to demanding performance requirements, such as mechanical strength and long term durability. Thus, there are significant restraints imposed on the design of many medical devices and/or their components.
As a particular example, mechanical heart valve prostheses include an orifice ring with one or more occluders. Commonly, mechanical valve occluders consist of two thin hemi-discs called leaflets, but other occluders include thick discs and balls. Heart valve occluders/leaflets perform the function of opening and closing to regulate the blood flow through the heart valve. Heart valve occluders typically pivot with each cycle of a pumping heart to open and close the valve at appropriate times. The heart valve prosthesis should provide good hemodynamic performance. In addition, the valve should be durable to provide stable performance over many years of use.
While mechanical valves generally provide important clinical benefits, these benefits are counterbalanced by the need for anticoagulation therapy and the associated risks of anticoagulant bleeding due to such therapy. Another limitation with some mechanical valves is a residual transvalvular pressure difference across the open valve that is larger than the pressure difference for a healthy native valve. An excessive transvalvular pressure difference imposes an extra workload on the patient's heart that may contribute to disease in cardiac tissue. Pyrolytic carbon is a preferred material for mechanical heart valves because of its relatively high thromboresistance and its durability. However, there are limitations in the shape and thicknesses of valve components made from pyrolytic carbon because it is a brittle material with moderate strength. These shape and thickness limitations may result in higher than desired transvalvular pressure differences.
In addition, a variety of other medical devices, such as orthopedic prostheses and dental implants, can advantageously be formed with pyrolytic carbon. Orthopedic prostheses can be used for hard tissue replacement, bone replacement and joint replacement. Similarly, a variety of dental implants are used to replace teeth due to loss from dental decay or disease. The material properties of pyrolytic carbon may impose limitations on the use of pyrolytic carbon for these other applications.